There are several types of gas-fired infrared burners being used in various manufactured products. These burners usually incorporate one of three design features. The most used and successful burner design employs a ceramic plate that contains apertures to allow the flow of the gas-air mixture to the surface for combustion. Also some types of porous ceramic can be used. The ceramic plate is usually about 0.500 inches thick and possesses relatively low thermal conductivity. The plate can also be manufactured from ceramic fibers such as a product sold under the Fibre Fax brand name. U.S. Pat. Nos. 3,277,948 and 3,561,902 to Best describe such a burner. The fuel input to these type burners is usually limited to about 350 BTUH/in2 of emitting element surface.
The emitting surface of gas-fired radiant burners can also be produced from metal. These types of emitting surfaces have usually been metal form or metal screens. The metal screens are woven from metal strands. Experience with using these types of burners indicates that they have limited life due to failure of the screen. Failure of the screen allows the flame to retrogress into the burner plenum resulting in flashback. Stress developed during the weaving process probably contributes to these failures. Also, since the screen provides for quenching of the flame on its surface, the size of apertures needs to be relatively small. Therefore, the diameter of the wire from which the screen is woven is limited. The small diameter of the wire limits the strength and resistance to thermal fatigue. When these types of burners operate on a generally continuous basis, frequent replacement of failed burners is required.
The other method by which gas-fired radiant burners operate is for the flame and hot combustion gases from a conventional port type burner to be impinged on a surface (usually ceramic) capable of emitting infrared radiant energy. This concept of generating infrared radiant energy is not as efficient as the surface combustion type of infrared burners. There are also other methods of generating infrared radiant energy by which the energy is not directly produced by the burner. U.S. Pat. Nos. 4,546,553, 4,785,552, 5,230,161 and 6,114,666 to Best describe this technology. This type of design technology can also be used to convert short wavelengths to longer ones as described in U.S. Pat. No. 6,114,666 to Best.
There are some limitations associated with each type of gas-fired radiant burner presently in use. The burner that uses ceramic as an emitter surface is the type most widely used in industrial and commercial applications. However, because the emitter surface is made from ceramic, these types of burners are fragile compared to metal. Also, the ceramic emitter is subject to failure if it is used in applications where it can become wet, such as in outdoor gas grills as described in U.S. Pat. No. 4,321,857 to Best. However, this type of burner has been successfully used in outdoor grills when the grill is designed to protect the burner from rain.
The ceramic type of infrared radiant burner is used in many successful products such as disclosed in U.S. Pat. Nos. 4,321,857 and 5,676,043 to Best, and in many applications it will continue to be the burner of choice. There are other applications where its limitations prevent its use. As an example, the burner will fail (flashback) if it is fired at an input greater than about 350 BTUH/in2. A typical burner with a ceramic radiation-emitting surface is disclosed in U.S. Pat. No. 3,277,948 to Best. Also, when these types of burners are over fired, incomplete combustion can occur.
Burners that use an emitting surface that employs a woven screen have not been reliable and usually have limited life in most applications of continuous use or where the burner is exposed to thermal shock through cycles of heating and cooling. Both the metal screen burner and ceramic type burners can fail when the input of fuel is increased beyond the ability of the surface to quench the flame, which results in retrogression of the flame into the burner plenum. Foam type of metal emitting surfaces can minimize some of the problems described, but they introduce new problems. Because of the porous nature of the material, it acts as a filter. Over time the surface will become clogged with atmospheric contaminates and the flow area through the surface is decreased resulting in variations in the combustion intensity over the surface. Also this type of material is expensive compared to other types of emitting surfaces. One type of this kind of porous metal is sold under the trade name of Metpore.
Another limitation of existing infrared burners is that when the primary air for combustion is supplied through a venturi as opposed to a pre-mixture of fuel and air supplied through a combustion air blower and mixer, secondary air for combustion is usually required. This phenomenon is notably true if the firing rate exceeds about 350 BTUH/in2 of burner emitting surface. Typical infrared radiant burners of this type are described in U.S. Pat. Nos. 3,277,948 and 3,561,902 to Best. When the input of fuel to infrared burners (described by U.S. Pat. Nos. 3,277,948 and 3,561,902) is limited to under about 350 BTUH/in2 of emitting surface, they can operate with 100% primary air with the use of a venturi. However, it is highly desirable in many applications to increase the energy input per unit area of emitting element surface and to distribute the energy systematically over the combustion surface of the burner. This is not practical to do with prior art type burners described above. Also, when an emitting element of a radiant type burner is placed close (within one inch) to an absorbing body, the emitting element temperature increases, thus increasing the tendency of prior art type burners to flashback. In many of the prior art type burners, secondary air for combustion is required. Some design restrictions are imposed in many applications when secondary air for combustion is required to ensure complete combustion. Also, secondary air for complete combustion is hard to control and usually results in excess air to the combustion process, which lowers the flame temperature and decreases combustion efficiency.
Another limitation of existing burner designs is that the emitting element is usually continuous. That is, the emitting surface area comprises most of the open side of the burner plenum. The emitting surface is usually surrounded by a border of about one half inch. In many applications of infrared type burners, it would be desirable to distribute the energy over larger surfaces than that of the emitting element itself. An example of such an application is the heating of the glass emitter described in U.S. Pat. No. 6,114,666 to Best. When it is possible to uniformly distribute the energy over the entire surface of the glass emitter, the burner can be placed very close to the underside of the glass eliminating the need to provide space for concentrated infrared energy to be dispersed over a larger area than its emitting area.
There are many other applications of the use of infrared radiant energy where it would be desirable to distribute the emitted energy over a larger area, such as in the curing of paint. There are other applications where it is desirable to concentrate more energy in a confined area than would be possible with existing technology where the combustion air is supplied through a venturi. Such an example would be to replace the conventional burner of a range top with a radiant type burner. It would provide many benefits if an infrared radiant type burner could have greater latitude in the amount of energy that is emitted over the surface of the burner—that is, for the firing rate to be dramatically increased or decreased per unit of area of the burner surface. Most of the prior art type infrared burners in use that use a venturi for the introduction of combustion air are limited to about 350 BTUH/in2 of burner surface when operating at high fire and the more normal high fire rating of these types of burners is about 250 BTUH/in2.